1. Field of the Invention
The field of the invention relates to threat detection systems generally and, more particularly, to an apparatus and method for calibrating a trace detection portal.
2. Description of Related Art
Extant threat detection systems check persons and objects for traces of substances of interest, such as narcotics and explosives. Such systems operate on the basis that trace amounts of substances of interest tend to be transferred to the body of a person who handles them, and from the person's body to any article the person's body may touch. Attempts have been made to test persons without physically touching them, but articles such as suitcases and handbags are tested by swiping them with a small piece of material that is then inserted into a known type of threat detection apparatus, which tests for the presence of the substance(s) of interest.
A trace detection portal is a known type of threat detection system into and/or through which a person can walk. U.S. Pat. No. 6,073,499 (the “'499 patent”) illustrates a known type of trace detection portal. A trace detection portal, such as the one described in the '499 patent, operates based on the principle that a person's body heats the boundary of air surrounding it, and that the heated air, being less dense than ambient air further away from the body, flows upwardly to create a thermal plume about the body. The rising thermal plume entrains particles comprising a substance of interest present on the person's body and carries them up and away from the body. A fan or other airflow generator positioned in a portion of the trace detection portal above the person operates at a speed that approximates the airflow rate of the rising thermal plume. The fan thus directs the thermal plume to a detector without drawing significant volumes of ambient air into the detector. Consequently, a significant concentration of particles comprising the substance of interest is created.
Some types of trace detection portals route the thermal plume directly to a detector for analysis. Other types first route the thermal plume through a trap that collects particles of interest from the thermal plume. The trap is then inserted within a desorber. Within the desorber, the trap is heated rapidly to temperatures of about 200 degrees Celsius to desorb and volatize the trapped particles comprising the substance of interest collected from the thermal plume. Clean air is injected into the desorber at a low rate and suction is applied to draw the clean air and the particles on the trap into the detector. The detector then detects and identifies the presence of the particles comprising the substance of interest. The trap will remain in the desorber until it is time for the next sample collection. The trap is then removed from the desorber, and repositioned across the airflow inlet at the upper portion of the sample collection chamber and rapidly cooled in preparation for the next sample collection. U.S. Patent Application Publication No.: 2004/0131503 illustrates such a known type of desorber and trap.
Calibrating a trace detection portal to accurately and consistently test for particles comprising a substance of interest is difficult, and typically involves misting differing amounts of a calibrant into the trace detection portal's sample collection chamber by hand. For example, calibrant can be delivered into the portal detection chamber using a hand-held container such as a pistol-grip sprayer, aerosol spray can, nasal spay bottle, etc. Delivering an effluence of calibrant into the portal detection chamber in this manner is imprecise for several reasons. First, the amounts of calibrant released will differ from person to person depending on how long each person actuates the hand-held container. Second, if the calibrant is released too far from the upper portion of the detection chamber, ambient airflow turbulence will reduce the concentration too much for calibration purposes. Other challenges include ensuring a trace detection portal is calibrated on a routine basis, for example, at the beginning of each shift, which may be once every eight hours of usage that the trace detection portal is used. Non-calibration can create regular periods during which the portal cannot be used. Such periods decrease the trace detection portal's throughput.
Long-felt needs thus exist for: an apparatus and method that can calibrate a trace detection portal automatically, simply, and accurately; an automatic and accurate calibration apparatus that can be easily retrofitted to existing trace portal detection systems; and a calibration apparatus and method for consistently dispersing a predetermined amount of calibrant into a sample collection chamber at a predetermined distance from a calibrant collection area of the trace detection portal.